One of the manufacturing processes of an integrated circuit is a photolithographic process for transferring a designed layout pattern onto a semiconductor wafer. In this photolithographic process, the pattern printed on a mask is transferred onto the semiconductor wafer using photographic technique.
If dust or other particle sticks on the mask in the photolithographic process, the dust or particle is also projected onto the semiconductor wafer. This may be a cause of producing defectives. In order to solve this problem and to prevent sticking of dust and other particles on the mask, a mask protective device is employed.
A conventional mask protective device comprises a rectangular frame, the size of which is set to surround the layout pattern of the mask, and a transparent thin film which is fixed to the frame around the periphery thereof in such a manner as to close one open end of the frame. The mask protective device is mounted on the mask by bonding the other open end of the frame to the mask with bonding means such as a pressure-sensitive adhesive tape.
When the mask protective device is disposed on and fixed to the mask in this manner, there is a space to be hermetically sealed, of which height corresponds to the height of the frame, between the mask and the transparent thin film disposed on the one open end of the frame of the mask protective device.
When the space i.e. the inside of the mask protective device is hermetically sealed, it sometimes happens that the thin film spread on the frame swells or dents due to changes in atmospheric pressure and/or in temperature. The swell or dent in the thin film may change the optical characteristics of the film and/or, in case of severely swelling or denting, may damage the thin film by that the thin film comes in contact with the mask or a rid of a casing which accommodates the mask and the mask protective device therein.
In order to solve the aforementioned problem, there has been proposed a mask protective device which has a frame provided with at least one vent hole which is closed by a filter to prevent entrance of dust or other particles and to allow the communication between the inside of the mask protective device and the atmosphere. This technique is disclosed in Utility Model Publication No. S63(1988)-39703.
However, the mask protective device in which the frame having the vent hole with the filter has the following problems: (1) the processes for forming the vent hole in the frame, periodically cleaning of the vent hole and the filter, and thus detaching and attaching the filter increase the man-hour in the manufacturing and also in the maintenance, and it is necessary to produce the vent hole and the filter in such a manner as to meet the quality standard; (2) there is a risk of dusting when blowing the filter to clean the filter; and (3) forming the vent hole in the frame loses the strength of the frame, in particular, the larger the number of vent holes and/or the larger the diameter of the hole to improve its permeability, the lower the strength of the frame.
It is an object of the present invention to solve the aforementioned conventional problems and to provide a mask protective device which allows the communication between the inside space and the atmosphere to provide permeability, and into which dust and other particles yet hardly enter.
It is another object of the present invention to provide a mask protective device which allows the communication between the inside space and the atmosphere to provide permeability without losing the strength of the frame, and into which dust and other particles yet hardly enter.
It is still another object of the present invention to provide a mask protective device into which dust and other particles hardly enter without using a filter, and which allows the communication between the inside space and the atmosphere to provide permeability.